1. Field of the Invention
The present invention relates to an image drawing system, an image drawing server, an image drawing method, and a computer program for providing three-dimensional virtual world to at least one client terminal.
2. Description of the Related Art
In keeping with the trend of three-dimensional Internet and three-dimensional games, there is an increasing demand by users to access three-dimensional virtual world from a low-performance device such as a mobile phone. However, it is difficult to transmit a large amount of image information on the virtual world to a low-performance device such as a mobile phone because of problems with the communication time period and communication traffic resulting from a narrow bus width for data transfer of a low-performance device. Further, it is also difficult to render a large amount of transmitted image information on virtual world three-dimensionally because of the processing power of a low-performance device. Many methods have been proposed for enabling access to the virtual world from a low-performance device.
Japanese Unexamined Patent Publication (Kokai) No. H10 (1998)-021215 (Patent Document 1) discloses a method of three-dimensional space imitation drawing in perspective based on a two-dimensional still image. According to the method described in Patent Document 1, first, a client terminal shrinks a still image showing an object or the appearance of an avatar in accordance with the depth of the arrangement thereof on a two-dimensional still image in perspective showing the background of the space seen from the viewpoint of each avatar. Then, the client terminal overwrites the image on the background image in the order of decreasing depth and detects the collision of mutual positions of the avatar or the object to control the movement.
Japanese Unexamined Patent Publication (Kokai) No. 2002-140731 (Patent Document 2) discloses a method of realizing smooth image movement even without the use of three-dimensional images and a lot of two-dimensional still image data. According to the method of Patent Document 2, an image processor conducts zoom-out or zoom-in processing with respect to at least one sheet of two-dimensional still image data representing virtual world so as to appear three-dimensionally in perspective, centering around a predetermined point on the two-dimensional still image data, where such processing is performed gradually while changing a magnitude, and a result of the zoom processing performed gradually is displayed one by one on a display screen of a computer or the like.
Japanese Unexamined Patent Publication (Kokai) No. 2003-511782 (Patent Document 3) discloses a method of decreasing the number of hours required for downloading a webpage. According to the method of Patent Document 3, a web server displays elements of a web page, such as a moving image or video stream data, as just still images, thus decreasing a size of a graphic image element and decreasing time required for downloading.
According to the methods described in Patent Documents 1 and 2, because a two-dimensional still image of the background within a range viewable from the viewpoint of the avatar is used, the server has to create a new two-dimensional still image every time the avatar's viewing direction changes even when the viewpoint does not change, and the client terminal has to receive the new two-dimensional still image to process it. According to the methods in Patent Documents 1 and 2, the movement of an image can be implemented by zooming in or out on an object or an avatar in the virtual world without the need for the client terminal to receive data for representing the movement of the image from the server one by one. However, in Patent Document 1, the movable range of the avatar is narrow and limited. In the method of Patent Document 2, when the avatar moves, the server creates a new two-dimensional still image while setting the position after the movement as a viewpoint, and the client receives the new two-dimensional still image and sets it as a background image. Thus, when a two-dimensional still image is transmitted to the client terminal every time the avatar moves, generating the additional processing thereof. Because of these problems, when the client terminal is a low-performance device such as a mobile phone, the three-dimensional virtual world cannot be provided thereto.
In view of the above-stated problems, it is an object of the present invention to provide an image drawing system, an image drawing server, an image drawing method, and a computer program that provides three-dimensional world to a client-terminal as a low-performance device.